"Motion study"
is a catch-all term for simulating and
analyzing the movement of mechanical assemblies
and mechanisms. Traditionally, motion
studies have been divided into two categories:
kinematics and dynamics. Kinematics is
the study of motion without regard to
forces that cause it; dynamics is the
study of motions that result from forces.

Other closely related terms for the
same type of studies include multibody
dynamics, mechanical system simulation,
and even virtual prototyping.

Kinematics is a simpler
task and may be adequate for many moving-parts
applications. Essentially, such simulations
show where all of the parts in an assembly
are in time as it goes through a cycle.
This technology is useful for simulating
steady-state motion (with no accelerations)
as well as in evaluating motions for interference
purposes, such as assembly sequences of
complex assemblies. Many basic kinematic
packages, however, go a step further to
provide "reaction forces," that
is, forces that result from the motion.

Dynamics simulation is more
involved in that the problem needs to
be further defined and more data comes
as a result of accounting for forces.
But, dynamics are often required to more
accurately simulate the real motion of
a mechanical system.

Traditionally, kinematics
and dynamics have followed the classic
analysis software method of preprocessing
(preparing the data), solving (running
the solution algorithms, which involve
the solution of simultaneous equations),
and postprocessing (analyzing the results).
Even though today's programs are much
more interactive, most follow this basic
process since it is a logical way to attack
the problem, and most solvers are a separate
program.

One of the reasons that
solid modeling is becoming so popular
is that it sets the stage for so many
applications. You can practically create
working drawings automatically, realistically
render models that look like the real
thing, and generate physical models from
rapid prototyping equipment. Similarly,
studying the motion of moving mechanisms
and assemblies is fast becoming almost
a "free" byproduct of solid
modeling, helping engineers to:

Produce animations for output to
video or posting on Web sites to show
customers and clients how your product
will really work and not just
pretty pictures of how it might
work.

The basic outputs are:

Animations of motion are
the classic output from a simple kinematic
analysis. The first use of such an
animation is simple a visual evaluation
of motion for the designer to see
if it is what is desired. More sophisticated
animations can be created to show
motion from critical angles or even
looking inside of parts, which gives
simulation a definite edge over building
and running a physical prototype.

Interferences
can be shown in a variety of ways.
Most systems will provide color feedback,
say, by turning parts that experience
interferences red. More useful, however,
are systems that turn the interference
volume into a separate piece of geometry,
which can then be used to modify the
parts to eliminate the interference.
Indeed, this ability to detect and
fix interferences without switching
between software is one of the primary
benefits of integrating motion simulation
and CAD.

Trace functions
provide more insight into motion.
The motion of a particular joint or
point on a part can be plotted out
in 3D space as a line or a 3D surface.
Or, the system can "leave"
copies of the geometry at specified
intervals. Such functions can also
provide an envelope of movement that
can be then used to design housings
or ensure clearances.

Motion data such
as forces, accelerations, velocities,
and exact locations of joints or points
on geometry can usually be extracted
as well, although such capabilities
are more applicable to dynamics simulations
rather than kinematic studies. Some
systems allow users to attach "instruments"
onto their models to make it simple
to specify what results they like
to see.

Plots and graphs of
such data are most commonly used since
the values vary overtime and are much
more meaningful than a single value
at any given time. Most packages provide
a plethora of plotting and graphing
functions. One especially useful capability
for studying design alternatives is
to plot the results of two different
simulations on the same graph. Such
data can also help designers determine
the sizes of motors, actuators, springs,
and other mechanism components.

Forces that result from
the motion are of particular interest
since they can be used as loads (or
at least to calculate them) for structural
analysis of individual members. Typically,
the highest load for a cycle is used
to perform a linear static finite-element
analysis (FEA) of critical individual
components in a mechanism. Integration
of solid modeling, motion simulation,
and FEA software can greatly streamline
this process, especially important
when studying design alternatives,
where many analyses are required.

Performing these tasks with
the aid of software is nothing new--specialized
programs for such analyses have been used
for years by engineers with a critical
need for such technology for projects
such as suspension design. But, doing
all of these tasks inside your CAD system
on an everyday basis is just now catching
on as solid modelers are being tightly
linked to motion simulation software.
Indeed, some feel that such capabilities
will become integral to solid modeling
and thus a part of almost every engineer's
tool box.